N2 - The present work aims at the characterization of aerodynamic noise from wind
<br/>turbines. There is a consensus among scientists that the dominant aerodynamic
<br/>noise mechanism is turbulent boundary trailing edge noise. In almost all operational
<br/>conditions the boundary layer flow over the wind turbine blades makes a
<br/>transition from laminar to turbulent. In the turbulent boundary layer eddies are
<br/>created which are a potential noise sources. They are ineffective as noise source
<br/>on the airfoil surface or in free flow, but when convecting past the trailing edge
<br/>of the airfoil their efficiency is much increased and audible sound is radiated.
<br/>We performed measurements of the boundary layer velocity fluctuations and the
<br/>fluctuating surface pressure field in two different wind tunnels and on three different
<br/>airfoils. The first wind tunnel is the one of LM Wind Power A/S following
<br/>the classic concept for aerodynamic wind tunnels with a hard wall test section.
<br/>Acoustic far field sound measurements are not possible in this tunnel due to
<br/>the high background noise. The second wind tunnel is owned by Virginia Tech
<br/>University. The test section has Kevlar walls which are acoustically transparent
<br/>and it is surrounded by an anechoic chamber. In this experiment the far field
<br/>sound was measured with a microphone array placed in the anechoic chamber.
<br/>The measurements were compared to predictions with an analytical model for
<br/>trailing edge noise. The analytical model is divided into two steps. First the
<br/>fluctuating velocity field is related to the fluctuating surface pressure field, then
<br/>the far field trailing edge noise is related to the surface pressure field close to the
<br/>trailing edge of the airfoil. The data base of measurements was used to evaluate
<br/>the different parts of the original analytical trailing edge noise model and to
<br/>improve it, because the predictions gave in general too low far field noise levels.
<br/>Our main finding is that the acoustic formulations to relate the fluctuating surface
<br/>pressure field close to the trailing edge of airfoil to the radiated far field
<br/>sound give excellent results when compared to far field sound measurements
<br/>with a microphone array and measured surface pressure statistics as input up
<br/>to a frequency of about 2000-3000Hz. The fluctuating surface pressure field
<br/>can be measured in a wind tunnel with high background noise due to the high
<br/>level of the fluctuating surface pressure field. Hence, trailing edge noise can be
<br/>evaluated by means of measured surface pressure field, even in cases where a
<br/>direct measurement of trailing edge noise is not possible. This opens up great
<br/>new vistas, i.e. by testing new airfoils in a standard industrial wind tunnel or
<br/>by testing new wind turbine rotors in the field.
<br/>The main difficulty for trailing edge noise modeling is to predict the fluctuating
<br/>surface pressure field correctly and one uncertainty of the original model was the
<br/>assumption of isotropic turbulence. This was investigated in the present work
<br/>and a new model to relate the boundary layer velocity field to the surface pressure
<br/>field accounting for an anisotropic turbulence spectrum was proposed. The
<br/>results were very similar compared to the original model and underestimated
<br/>the measured one point surface pressure spectrum, even though the prediction
<br/>of the one point velocity spectra was improved.

AB - The present work aims at the characterization of aerodynamic noise from wind
<br/>turbines. There is a consensus among scientists that the dominant aerodynamic
<br/>noise mechanism is turbulent boundary trailing edge noise. In almost all operational
<br/>conditions the boundary layer flow over the wind turbine blades makes a
<br/>transition from laminar to turbulent. In the turbulent boundary layer eddies are
<br/>created which are a potential noise sources. They are ineffective as noise source
<br/>on the airfoil surface or in free flow, but when convecting past the trailing edge
<br/>of the airfoil their efficiency is much increased and audible sound is radiated.
<br/>We performed measurements of the boundary layer velocity fluctuations and the
<br/>fluctuating surface pressure field in two different wind tunnels and on three different
<br/>airfoils. The first wind tunnel is the one of LM Wind Power A/S following
<br/>the classic concept for aerodynamic wind tunnels with a hard wall test section.
<br/>Acoustic far field sound measurements are not possible in this tunnel due to
<br/>the high background noise. The second wind tunnel is owned by Virginia Tech
<br/>University. The test section has Kevlar walls which are acoustically transparent
<br/>and it is surrounded by an anechoic chamber. In this experiment the far field
<br/>sound was measured with a microphone array placed in the anechoic chamber.
<br/>The measurements were compared to predictions with an analytical model for
<br/>trailing edge noise. The analytical model is divided into two steps. First the
<br/>fluctuating velocity field is related to the fluctuating surface pressure field, then
<br/>the far field trailing edge noise is related to the surface pressure field close to the
<br/>trailing edge of the airfoil. The data base of measurements was used to evaluate
<br/>the different parts of the original analytical trailing edge noise model and to
<br/>improve it, because the predictions gave in general too low far field noise levels.
<br/>Our main finding is that the acoustic formulations to relate the fluctuating surface
<br/>pressure field close to the trailing edge of airfoil to the radiated far field
<br/>sound give excellent results when compared to far field sound measurements
<br/>with a microphone array and measured surface pressure statistics as input up
<br/>to a frequency of about 2000-3000Hz. The fluctuating surface pressure field
<br/>can be measured in a wind tunnel with high background noise due to the high
<br/>level of the fluctuating surface pressure field. Hence, trailing edge noise can be
<br/>evaluated by means of measured surface pressure field, even in cases where a
<br/>direct measurement of trailing edge noise is not possible. This opens up great
<br/>new vistas, i.e. by testing new airfoils in a standard industrial wind tunnel or
<br/>by testing new wind turbine rotors in the field.
<br/>The main difficulty for trailing edge noise modeling is to predict the fluctuating
<br/>surface pressure field correctly and one uncertainty of the original model was the
<br/>assumption of isotropic turbulence. This was investigated in the present work
<br/>and a new model to relate the boundary layer velocity field to the surface pressure
<br/>field accounting for an anisotropic turbulence spectrum was proposed. The
<br/>results were very similar compared to the original model and underestimated
<br/>the measured one point surface pressure spectrum, even though the prediction
<br/>of the one point velocity spectra was improved.